Marshmallow creme is scoopable, spreadable marshmallow confection. Packaged in jars, the semi-liquid marshmallow is much softer than a regular marshmallow, and both creamy and sticky at the same time. I tend to always call this confection “marshmallow fluff,” although the name might not be familiar to all consumers, as it is actually a trademark for a single product. When produced by other companies, the marshmallow goo is labeled as “marshmallow creme” and you’re much more likely to see it on store shelves that way.
Marshmallow creme was invented in the early 20th century, after “regular” marshmallows had been in production for a number of years. It is made using some of the same ingredients that come into play in most marshmallow recipes – sugar, corn syrup, vanilla – but creme usually uses egg whites (often in powdered form) while regular marshmallows tend to use gelatin as a stabilizer. The reason that marshmallows usually use gelatin is to give them some firmness, allowing them to be sliced and packaged in pieces. Marshmallow cream does not firm up and remains soft, even when it sits out for a while.
There are plenty of recipes that use marshmallow creme, including Fluffernutters and S’mores Cookie Bars. You can make a great cream cheese frosting with the fluff, too. You can’t however, substitute regular marshmallows for fluff even if you melt them down. Fluff keeps for a very long time in the cupboard, so it’s something to keep on hand for a time when you might need it even if its not on your regular grocery list.